expressions be selected that best approximate
the actual measured data.
Thus, the equations for VO and RS can be written
as:
For operation over a restricted range of current,
say from 30 mA to 70 mA, the forward current
can be modeled with a linear model. As shown
in Figure 3.14, the linear model draws a straight
line between two points (IF1, VF1) and (IF2, VF2) at
two forward currents, IF1 < IF2, to linearize the
electrical forward characteristics between these
forward currents. The linear model is shown
graphically in Figure 3.15 for the forward voltage
versus forward current curve shown in Figure
3.7. The equation for the forward current
becomes:
For most applications this linear model can be
used to model the forward characteristics of an
LED emitter. For best accuracy, the use of the
linear model should be restricted to a range of
forward currents, IF2 / IF1, less than 4:1. For
operation at a lower range of currents, different
points (IF3, VF3) and (IF4, VF4) can be selected to
bracket the approximate range of operating
current. However, it’s always important to
recognize that the linear model only works for
a specified range of forward currents (IF1 £?IF
£?IF2) as the accuracy of the linear model
degrades quickly outside of this range. It
should go without saying that the linear model
cannot be used at all for values of VF < VO.
Where:
VO = turn-on voltage, the y-intercept of the
straight line (IF = 0)
RS = series resistance, the slope of the straight
line
Figure 3.15 Linear Forward Voltage Model for
HPWA-xHOO LED Emitter Shown in Figure 7.
Figure 3.16 Worst-Case Linear Forward Voltage
Models for LED Emitters.
17
LUMILEDS [ LUMILEDS LIGHTING COMPANY ]
